The invention relates generally to environmental cooling systems based on regenerative desiccant dehumidifiers and more particularly to the means by which the desiccant is regenerated or restored.
Desiccant based cooling systems have been known for some time to have potential as heat-actuated space-cooling devices. Desiccant systems operate entirely on heat and mass transfer processes. An open cycle desiccant cooling system is essentially a hybrid of two fundamental air processors: a desiccant dehumidifier and an adiabatic evaporative cooler. In the simplest embodiment, outdoor ambient air passes through a drying wheel of hygroscopic material which absorbs or adsorbs moisture from the air, accompanied by heating of the air. The dried warmed air then flows through a heat exchanger, usually a second rotating wheel, where it is cooled by the transfer of sensible heat. The dried cooled air is then further cooled and reconstituted to a desired humidity by passing it through an evaporative humidifier. In the desiccant wheel, the dehumidification process converts the latent heat of water vapor to sensible heat by means of absorption or adsorption. Thus, the desiccant wheel is sometimes referred to as the L-wheel while the heat exchanger, if rotational, is referred to as the S-wheel, alluding to the transfer of sensible heat.
There are numerous ways to configure the layout of these various components, namely, the L-wheel, S-wheel and evaporators, in order to modify or improve performance characteristics. In all of the systems, however, one of the main features is the regeneration of the desiccant. Typically the L-wheel is divided into a process side and a regeneration side by means of diametrical and circumferential seals. Heated air is blown through the regeneration side to dry out the desiccant so that when it rotates into the process side, it is available again for sorption of water from ambient air.
Open cycle desiccant cooling systems based on natural adiabatic and heat transfer processes offer a set of characteristics which make them potentially more cost effective than vapor compression electric air conditioning systems in certain applications. For example, larger roof top installations represent an excellent application, particularly where ventilation of the building is also a requirement. Here, the lower energy consumption and lack of high pressure coolant lines and seals make desiccant cooling systems attractive, especially where added heat for regenerating the desiccant is provided by gas burners, gas costing much less than electricity in the summer.
As with any energy related product, however, one of the focal points for cost effectiveness is high performance component design. From a system standpoint the desiccant regeneration system is quite important in terms of increasing the overall efficiency or so called coefficient of performance ("COP"), (namely, the cooling capacity divided by the thermal energy input of the cooling system) without sacrificing the specific cooling capacity ("SCC") (BTU/lb.sub.da). The rotating L wheel, divided in two by sliding diametrical stationary seals, exhibits a static angular temperature profile or gradient which is approximately symmetrical with respect to the plane of the partition between the process and regeneration sides. At any given moment of operation after equilibrium is reached, the wheel is hottest where it leaves the regeneration side and enters the process side and declines in temperature to the coolest region where it leaves the process side and enters the regeneration side. In general, in prior art systems, air on the regeneration side leaves the heat exchanger (S wheel) in a uniform, mixed stream and is relatively uniformly heated by the regeneration heater before passing through the desiccant wheel. The temperature of the regeneration air immediately prior to entering the desiccant wheel is approximately the same throughout the semi-circular area.